Electromagnetic wave microwave frequency filter



W. D. LEWIS Deg. 22, 1953 ll Sheets-Sheet .1

Filed Feb. 21, 1951 FIG.

PROTOTYPE M/CROWA VE F/L TER OUTPUT R A n M W m wwmimmqw P SE3 m, Y A MM M fi 0 mmmimmqv Runs: I n n A m 1 R 208mm 535 FIG. 2

IDEAL /z0 ourpur REGION OF A MICROWAVE FILTER lA/I/ENTOR W. D. LEW/S A TTOPNEV Dec. 22, 1953 W. D. LEWIS ELECTROMAGNETIC WAVE MICROWAVE FREQUENCY FILTER Filed Feb. 21, 1951 AMPLITUDE vs FREQUENCY/N MICROWAVE FILTER w/ TH UNIFORMLY lLLUM/NA r50 DELAY MAN/FOLD 11 Sheets-Sheet? AMPLITUDE V5 FREQUENCY /N M/CROWAl E F/L TER WITH DELAY MAN/FOLD lLLUM/NA TED BY COS/NE AMPLITUDE TAPER I A TTORNEV W. D. LEWIS Dec. 22, 1953 ELECTROMAGNETIC WAVE MICROWAVE FREQUENCY FILTER ll Sheets-Sheet 3 Filed Feb. 21, 1951 l 0 X 86 3% m2 A TTORNEV 11 Sheets-Sheet 4 W. D. LEWIS ELECTROMAGNETIC WAVE MICROWAVE FREQUENCY FILTER n N \\\q M 4 |||||l\l.|\\\\ 2 4 2 W F |Ilfl F 0W A a 1111/ E C 9 0 c V 8 N 9 ND 0 E G 0 U G 1 F u l 0 F G W m m F w E F ,R k 1% am I F F 2 F/ w MHIIHHIIIHUIIRHI 1 L 111V 1 W F m P F 9 III!!! v 1/ m fiqqtw 3K co k2, w ME 3m 29.6mm ot no matter M. E 5E5 \GGQSQ E MM x m3 FR w 3 T m3 8% ll. a m3 wfibn o 39 35: NR KR 3 #35 m w? .GRSQ wk m Quv mzkmomw v MR \E 7 RR \R snow. R C a awn owk av p US$39 mom a? 5R qt ms 8:955 \fiflnSQ 86 Dec. 22, 1953 Filed Feb. 21, 1951 W. D. LEWIS Dec. 22, 1953 ELECTROMAGNETIC WAVE MICROWAVE FREQUENCY FILTER Filed Feb. 21, 1951 ll Sheets-Sheet 5 wk 9% R 9% lNl/ENTOR W D. LEW/S BY 7% Q I ATTORNEY W. D. LEWIS Dec. 22, 1953 ELECTROMAGNETIC WAVE MICROWAVE FREQUENCY FILTER Filed Feb. 21 1951 ll Sheets-Sheet 7 7%QWW ATTORNEY W. D. LEWIS Dec. 22, 1953 ELECTROMAGNETIC WAVE MICROWAVE FREQUENCY FILTER Filed Feb. 21, 1951 11 Sheets-Sheet s 0 v3 8 mom amt I w; wiomui qzbooiou INVENTOR W. D. LEW/S ATTORNEY W. D. LEWIS ELECTROMAGNETIC WAVE MICROWAVE FREQUENCY FILTER ll Sheets-Sheet 9 lNl/ENTOR W.D. LEW/S #92 W I ATTORNEY Dec. 22, 1953 Filed Feb. 21, 1951 Dec. 22, 1953 w. D. LEWIS 2,663,848

ELECTROMAGNETIC WAVE MICROWAVE FREQUENCY FILTER Filed Feb. 21, 1951 11 Sheets-Sheet 10 FIG. 7

" 45 WAVE #00 oururs POSSIBLE DE 5 IGN TE C HN IQUE FOR MICROWAVE F/L TER ELECTR/C VECTOR Lav 7% A TTORNE Y Dec. 22, 1953 w. o. LEWIS 2,663,843

ELECTROMAGNETIC WAVE MICROWAVE FREQUENCY FILTER Filed Feb. 21, 1951 11 Sheets-Sheet 11 FIG. I85

OUTPUTS I605 T0 /809 DOUBLE REFLECTOR A /soa lNPUT woe INVENTOR W. 0. LE W/S fi m WW Patented Dec. 22, 1953 ELECTROMAGNETIC WAVE MicRoWAVE.

FREQUENCY FILTER v Willard D. Lewis, Little Silver, N. J assignor to Bell Telephone Laboratories, lncorporateiNew' York, N. Y., a corporation of New York Application February 21, 1951, Serial No. 212.129v

Claims. (01. 333-73) This invention relates to novel electromagnetic wave filters for use at very high or microwave frequencies. More particularly it relates to novel microwave frequency, electromagnetic wave, filters in which refractingor difiracting microwave frequency structuresare employed in combination with terminal microwave frequency arrangements, which combinations serve, for example, to effect the separation of theseveral frequency bands of multichannel radio microwave frequency system.

Optical prisms andgratings, classed generical- 13* as spectroscopes, are well known to those skilled in the optical art andare known to effect the. separationin space of a composite light beam into regularly arranged bands of differing wavelength (color) or frequency. Somewhat analogous effects can be obtained at microwave radio frequencies, but to obtain structures of, practicable physical dimensions itwill not sufliceto simply scale up. known optical spectroscopes,

since these usually have dimensions which are in the order of hundreds of thousands or even millions of wavelengths. Furthermorasuitableterminal arrangementsv for electromagnetic wave microwave frequency systems, which differ markedly from. theterminal arrangements for optical devices, are required, as willbecome apparent hereinafter. v

At microwave electromagnetic wave or radio frequencies ,(i. .e. frequencies above, say 100 megacycles) retracting or diffracting materials and/or structures can be employed which change their transmitting characteristics rapidly with frequency and thus structures of practicable physical dimensions can be obtained.

A principal object of the invention is, therefore, to provide novel electromagnetic wave structures for use at microwave radio frequencies which will effect the physical and electrical separation of a broad band of frequencies into a plurality of discrete subportions or frequency channels, each of which subportions is suitable for a communication channel to transmit, for example, a, television program video signal or a group of carrier. telephone or telegraph channels.

A further object is to provide novel types of microwave radiowave filtering devices. Other and further objects will'become apparent during the course of the following detailed de scription and from the appended claims.

The principle of the invention will be more readily understood .from'the following descrip- Ltion .of specific illustrative embodiments shown in theaccompanying'drawings in which: 7

, filter of the invention for Fig. 1 is a diagrammatic showing of certain fundamental featuresofa prototype electromagnetic microwave filter'of the invention;

Fig. 2 is'a diagrammatic showing of an idealized output region of a, filter of the invention;

Fig. 3 is a frequency versus amplitude transmission characteristic for a filter of the invention;

Fig. 4 is a group of characteristics for a group i of filters of the invention quency bands;

Fig. 5 shows in diagrammatic form ajspjecific configurationfor the output region of a, filter of the invention; v

Figs. 6A and 6B are plan and side views respectivelyof one specific example of a'filter of the invention; r

Fig. 7 is a further specific example of a filter structure of the invention involving double utilization of substantial portions of the structure;

Fig. 8 shows phase-versus-frequency characteristics employed in explaining particular filter structures of the invention.

Fig. 9 shows improved amplitude-versus-frequency characteristics which can be obtained by specific structures of the invention; j

Fig, 10 illustrates a further specific form of filter of the invention incorporating more complicated devices in the delay manifold of the Fig. 11 illustrates a composite or conjugate passing adjacent frecombination of twdstructure's of the invention to afford improved filtering characteristics;

Fig. 12 illustrates frequency-versus amplitude characteristics which can be obtained by arrangements of the type shown in Fig. 11;

Figs. 13 and 14 show specific composite or conjugate filter structures of the invention;

Fig. 15 illustrates a way of further increasing the utilization of substantial portions of a, specific filter structureof the invention;

Fig. 16 is a diagram employed in explaining the operation of the device of Fig. 15

a Fig. 17 is. an electromagnetic wave filter of the invention for use at extremely high frequencies;

Figs. 18A and 18B aretopand side viewsof a V useat extremelyhigh frequencies in which th double utilization of subistantial portions of the structure is efiected an Fig. '19 is a further modification of the-structure of Figs. 18Aand 183 wherein the degreeof utilization is still further increased.

In the first category of electromagnetic wave microwave. filters of the invention to be discussed, principles somewhat analogousgto those "Let us evaluate the fraction ofthe total energy distributing feed, for example, assume it is similar to the feed of a microwave antenna, which distributes power to the 2n+l separate input apertures, numbered, -n, (n1), 2,1, 0, l, 2, (n-l) n. Let the total input power be designated by P, and let the corresponding power which appears in the s'th input aperture be designated by PS. Let us designate the corresponding quantities for the output device and the s'th output aperture by P and PS, respectively. We define these in the same way,i. e., for P incident in the output we would observe P8 in the s'th output aperture, however, for power fiow in the direction weare considering, this means, by the law of reciprocity, that if there is power P3 in the s'th output aperture flowing towards the output, then, in the absenceof other waves, that P will appear'in the output. We are now in a position to compute the transmission from input to output. 1 a

Let V be the voltage at the inputs and 5 the phase of the total path from input to output through the s'th path. Then the voltage magnitude into the s'th aperture is given by,

and the resulting voltage magnitude from this into the output is so that the total voltage into the output is given by In other words, the efiect of transmission through the filter is represented by the factor:

The condition for no loss is that the magnitude of this factor (a/l) be equal to 1. This will occur if, and only if,

8 71 S=TII P ;=P E P,=P' (11.5) s=-n s=n (3) There must be no ohmic or reflection losses.

(4) The phase of all paths between input and output must be, effectively, equal (we must remember in this connection that the addition or subtraction of an integral number of wavelengths, i. e., of multiples of 21r to the phase, leaves it, effectively, unchanged).

Any departure from the above conditions in a practical microwave filter of the invention will result in some loss.

-6 "(b)" i Determinations in" a particular: case of. the variation of amplitude 1 with frequency in a microwave filter of the invention Let us begin by considering a somewhat idealized output region,lor section, Be, as illustrated diagrammaticallyin Fig. 2. ,The output section illustrated diagrammatically in Fig. 2 is substantially the same as that of Fig. 1 between the row of output'orifices A9 and the output device 0, except that in Fig. 2, output device 0 is displaced from the normal 200 to the -0 aperture, as shown. The row of 2n+l output apertures A0, is placed along a straight line at intervals d centerto. center, between adjacent apertures andthe foutputis placed at a large distance D from the centeraperture at an angle a from the normal 200 to the line of apertures. The distance of output device'O from the s'th aperture is readily'seen to' be approximately D-Sda. Let the phase of the component due to the s'th aperture be r 1% SkAf. where Af=ffo and fo is some reference frequency, i. e., let it be a constant plus a linear variation with frequency proportional to the position of the aperture in the line.- Let" the amplitude at the output due to the wave in the s'th aperture be proportional to cos 65 where a is a small angle, i. e., let the amplitude across the apertures have a slow variation with-a maximum at the center aperture. The voltage in the 21rD 21r v Awe...

. V (b.-1) The external phase factor does not affect the magnitude of this sum. The sum can be evaluated by .means of conventional mathematical methods and has the value .1 f varies approximately linearly with frequency. It followsthat a succession. of outputs at various angles can be used to divide up the band of frequencies in the neighborhood of f0 into a number of component bands.

.. runways ei piu sthe am litude ta er mos-s the aperture: m a: oglemersi ia notaper nata between ralle at s a and the-above funetion reduoesatoi coaxialylina or awave guide (In a waveguide. h i el v p re rsemmn r ctlysc n tant; b sin, r 2 X ni h n m renew? se -v i l oueh n 5 smt atr he ameee iem o era smay be iinfX V applied-, toa. gooclapproximation) which ,in the vicinityj fj isjapproxirnat rily The wavelength-inanyof the casesrlisted above q a s tomay; e wri te ni he m sin-lif 7" K lOi- .g-- ""\/-1-.- (9- where" K-;;(;2n +,1)- X;- where he isthe cut-off frequency wavelength (7\c=9 -foriree-spaee or coaxial line). The phase tine-to theinsertion of a length of: lineiis therefore Thi is a uns i e e yg amil ar m ne eskilledt he a enna ti spleite r' n. ie. .asj ur m; 7

Q 22 M=1n here sanefiee ivec s n am- L.". plltude' eller e e s" t e; utput pe ures. n M he" a ove; un i eni, r dueesl p oxi ely. to 71fwe useeTaylorfsrTheorem to texpandgthis; about This function is jplottdasthe solid curve i' 0 in where f Fig 4 and-is seen to have a-sornewhatbroader 1 1 main lobe and-smallersubsidiary lobes tha-m the 2 1 prior above-deseribed fpnctionplottegi as curve 1 x/1 E) QT-Of T- i M f In; orderto achieve frermene'y d-isoriminaition velocity oflight', by, means of a miorowavejfilt'erofthe invention v with an output curye;- l WOTZW as shown inFigPii (d) Deszgn Oj'the delay mamfozd or '4; respectively, we merelyneedi to en'l-ploy a In section (b) we assumed that the phase of the number of outputs displaced in angle--so 'tliat' component due to the s'th aperture was their maximum re sponses are satisiactorily clis- S=+SRM (hm placed in frequency. The responses in z the various additional outputs can then be, for exanmle, The results of ey r v us 586151011 ow that 111 asjinqigqted by curygsll 24 1 2. 4 inclusive, in g order to obtainthis result wecan choose the ef- 4 festive length of the path-'which produces this The basic prinegples of the simplest microwave Xm @151. filters of the present invention have been dis- L =L+sLa-- (d=.0) mi ed a q Ma y, ther mime r, t ure l m amplitude tanermg imvqllvingthejyuseuof Th s is equlvalentto saying-that the manuold 1s 'e ma b j yw zf yf to t'hd' r kiudz; constructed of an assembly oglines each 0ne of in the' art: Meie complex microwave filtersof i 1%? then. Its h r on one We the present invention-=-will be deseribed -herein- 342d. e ha its nfilghboron'thevother under. side L From Equation 0.2 we found that (e) Delay method of generating p ropep phase; 5 y

e r ie mir aw eu sl=e -r +m] In-the previous section, we assumed 'that the phase in the component due to the stlgaperfrom wmch'lt'fqnows that ture in a microwave filter of the invention was 2m(L+sL.a') fa. zrK H- La) d 1 equal to +skAf, where AI=j- +;fo, and in is some/ C 1 0 reference frequency; Inother words, we assumed 7 l c v h t v e be anaaqrzmem e hemes: varied Inmls-exire-ssmme Set approximately linearlywitlifrequency; ,ZJ K,(L+ L1)"JQ 3 t oalways true in grating-typeoptical spectroscopes e y and can be realized in a large olass of somewhat analogous microwave filter-structures. We shall by Ieqmn'ng that e h we r n Me ereing tha rdseer k from this conditiQp, makes possible further classes i iet wave fil ers e helinwetw I 1 is an integral multiple-of 21 It 15 well known to those ea nz Ila-l mme The-term in this equation-which contains the quency, lumped-element electrical network and roduct gAf is then t only one which t circuit that a ine r Variation Over 3 included inthesummation bal. Thisispredetermined range offrequency can be obtained by inserting a network or a transmission gL -ip f;

line has a constant delay tlrroughoutithe predeterminedf'frequeney range. hetfus assume -and-- can--be-setequal toiksaf of 11.0. It;iollows that thefphase charaeteristimof the-{component that du to the sth aperture is obtained -by 'in srt-irig (or eifectively insertingg -a transrrrission li rieof k=,2lL;,. (d.3) leneth lai This- =li-ne may*be-embodied asafree 7 c It follows also, from e.5 that the direction of maximum radiation is given by From the foregoing discussion we know that in this equation a is the angle, in radians, of the direction of maximum output from the direction at reference frequency, Ls is the length by which each line of the delay manifold is successively different from its neighbonrrl is the oenter-tocenter spacing of the output manifold and u is the quantity (6) Illustrative design of one form of microwave filter of the invention In the idealized output region of Fig. 2, it was assumed that the distance Dv was large with respect to the other dimensions. This made possible a simplification of the mathematics; but if D is made too large, it would result, in practice, in a physically impractical device. It can be shown, however, that ifD is made a length comparable to the length of the row or output apertures that approximately the same mathematics will apply, provided due allowance is made for fixed differences in phase.

By way of illustration, in the diagram of Fig. 5, a row of ouput apertures to 552, inclusive, of a manifold (not shown in Fig. 5) are arranged on an arc, the center of curvatureof which is oentrallyloeated on a corresponding are on which output feed devices 528 to 525,inclusive, are loeated. Conversely, the center of curvature of the are on which device are to 525, inclusive, are located is centrally located on the first-mentioned arc locus of the. apertures. The length (vertically) of each row is A, and the separation between the center points of the two above-described arcs is D. I v

Let us confine theoutput section to the region between two conducting parallel plates. In particular, we can, as described above, place the output apertures along the arc of a circle with its center of curvature at the junction of output feed devices 522 and 523, as shown. in Fig. 5. All the output feed devices can be-placed along the arc of a circle "with its centerof curvature at the junction of the centraloutput apertures 5&5 and 5M. This configuration will follow the above mathematical treatment particularly closely.

Let us now assume that we wish to obtain m chann ls spaced at, center-to-center, where moo) is small compared to-the reference frequency f0. ljhen, by Equation 01.4, the angular separation between successive ou'tput feed devices must be D g A (e.3)

is, therefore, a possible choice. The aperture array will therefore subtend an angle of approximately one radian, or say 60 degrees, at'each output feed device.

In order to avoid spillover losses (item 1, section a) the output feeds must be so designed that their radiation patterns fall almost entirely on the aperture array. From elementary antenna theory We see that this will occur if the feed has dimensions of'about 1.5x.

Let us place the output horns contiguously so that the total arc length occupied ;by them is a minimum and is equal to 1.5 mt. Let us make the total arc length occupied by these horns equal to the arc length of the array of apertures so that A g 1.5 mi (6.4)

Combining e.2, e.3, and e.4 we obtain Eel. 1

which fixes Liz/d. If we make d too large, La will become too large, the directivity of the output apertures will become too greatand, worst of all, we run therisk of obtaining spurious principal maxima inv expressions of the type'b.2. On the other hand if we make at too small many extra individual output apertures with an associated delay line for each one will be required and the filter willbecome unnecessarily complex. Simple antenna theory can be used'to show that for an output region of the proportions illustrated in Fig. 5, a value of d between 0.5x and A will produce a practicable structure which will operate satisfactorily. Let us assume, for example,-

d 0.757\ It now follows from e.5 that (a6) Here, F is a frequency equal to the totalfrequency range handled by the filter, and A is the wavelength corresponding to this frequency.

The objective of this section has been reached. It has been shown that we can build a microwave filter of the invention in which-the parallel plate output region or section is substantially square and has, in the plane of the'manifold, two dimensions of about 1.5 mt each, where m is the number of outputs and where there are 2m separate delay lines difiering in length by (1) Additional illustrative configurations In the foregoing section (6) we employ the theory developed in the previous sectionsia). .to (d), inclusive, to show how the parameters of ja microwave filter of the invention might be chosen when some of its performance requirements are specified. From the curves of Fig. 4, we seethat acesn flatter-topped and steeper sided amplitude-versus-frequency characteristics 'than "those obtainable at relatively lowtrequencies by singly resonant circuits in ordinary lumped-element circuit filtertechnique cannot .be expected from this elementary microwave filter technique. On the other hand, where the total frequency range to be handled is not too large .a fraction of the midband frequency and where a number'of approximately evenly spaced output channels are required, it isquite easy to specify an output region or section and a delay manifold consisting of transmission lines which will be adequate to produce the desired performance.

It cannot be emphasized too stronglythat the particular dimensions employed in the previous section and inconnection with :Fig. are for the sake of illustration only. 'Many other relative values of dimensions and many substantially different configurations can be employediwhich will embody the same general principles of the invention. Some of the large number of possible variations will be discussed below by way vof further illustration.

The particular dimensions indicated-in the previous section (6) and in Fig. 5, though not absolutely minimum, are in the smaller practical ranges. As the frequency is increased and the wavelength becomes smaller, it will obviously become advantageous to use dimensions which are larger in terms of the wavelength. The dimensions suggested in connection with Fig. 5 can, for example, be increased in several ways.

(a) The output feed devices can be increased in size provided that the distance D is increased in proportion. (This is permissible since this does not alter the angular position occupied by each feed and, at the same time, increases its linear dimensions so that its illumination pattern remains confined principally to the aperture array.)

(1)) The apertures and'consequently the aperture array can be increased in size provided that the distance D is increased in proportion. (As this is done, the rateof change'of angular deviation with frequency is decreased'in exact proportion to the decreasedangles of the output devices or horns.)

(c) Any desired combination of (a) and (b) above, can be employed. By way'of a particular example, if both arerincreased in the same proportion, the quantity D must be increased in proportion to the square of the factor of increase. In general, D must be increased in proportion to the product :of thetwo factorsof increase.

Throughout the above discussion, no mention has been made of the distanceseparating the parallel plates'of. the output region Where the electrical vector is perpendicular to theconducting' plates so that the transverse electromagnetic mode only is present, thisdistance cargin theory, be anything; If itismade too small, excessive ohmic loss will result. If it is'made greater than one-half wavelength, undesired modes can be propagated and mustbe guarded against. Modes other than the transverse electromagnetic may be used in this region-provided that the output feed devices or horns and the apertures are designed appropriately, in accordance with principles well understood by those skilledin the art. The parallel plates may. be omitted without changing the fundamentalprinciples of operation. In this case,.some.other.means of employ.- ing substantially two-dimensional energy transmission in a three-dimensional region can be used '12 (c. f." many "cylindrical microwave antennas. For example, a large cylindrical 'a-ntenna is' illustra'ted in Fig. 26 atpa'ge 262 of the paper Radar Antennas by H. T. Friis and "W; D. Lewis, Bell System Technical Journal, vol 26, No. 2 for April 1947. This antenna, obviously, would be of reasonably small dimensions ifascaled to a frequency of 50,000 megacycles or higher.)

Of more importance, perhaps, is the-case where the delay manifold itself is omitted and the input and output regions are so arranged that the distances involved from input feed to apertures. to output feed provide in themselves the proper delays to give the desired-effects. This, broadly speaking, is the principle of optical grating spectroscopes now in use.

Another important: case is where a dispersive region (one whose phase characteristics vary with frequency) is shaped into a prism and used as the delay manifold. This is analogous to the prism spectroscope of optics. These techniques become important and practical as soon as the wavelength becomes so shortxthat techniques approximating opticaltechniques'can be said to be applicable. In the foregoing, we have omitted a detailed discussion of the input region because none seems to be necessary. We can use any meansof distributing theinput power as desired among the various lines 'or regions of the delay manifold. In particular, wecan employ a waveguide manifold or wecan employ an open region in a manner similar to that suggested in connection with the output region. We can in some designs, illustrative examples'of which will be described in detail hereinafter, employ a region which is coincident with the output region.

A number of illustrative embodiments of the above-mentioned principles, structures, and techniques will be provided below.

In Figs. 6A and 63 a straightforward application of many of the'princ'iples described above, is shown in the form of a specific design of a microwave filter of the invention proportioned to operate over the frequencyregion between 45,000 megacycles to 55,000 megacycles, inclusive.

Fig. 6A is a top view, and Fig. 6B is a side view of the microwave filter. Device 609, as shown in Fig. 6A, is an input device comprising an electromagnetic horn, the throat section, or left end, of which is a section of wave guide of rectangular cross section having inside dimensions of inch by g'inch', the larger dimension appearing in Fig. 6A. The right endof horn 600 has a flare in the plane of the'paper, Fig'BA, such that continued, as sides 632 and 634 of the input section, they meet the outermost sides B42 and 644, respectively, of the delay manifold and the output section both of which last two mentioned portions will be described in detail'presently. The longitudinal axis of horn- B00 is coincident with the longitudinal axis or center line Bill of the entire structure.

"The enclosureof the input region 630 is completed by parallel'plates 636, 638, shown as top and bottom boundaries 'of the region, in Fig. 6B. The distancebetweenplates636 and 638 is 3% inch, internal dimension.

The delay manifold M comprises 20 lengths of wave guide designated to 620, inclusive, respectively, each wave guide being of rectangular cross section and having internal cross-sectional'dimensions'of es inch by inch, the inch dimension (top) being shown in Fig. 6A and the inch dimension (side) being shown in Fig. 6B.

. l3 The shortest wave guide GUI is madeapproxi mately two inches long and includes onlya small loop, if any. Each highernumberedwave guide is approximately 1.3 inches longerthan the next lower numbered wave guide. The precise lengths of all the wave guides are chosen "so'that the energy components emerging from the right ends of all the guides are exactly in phase at'the midfrequency, 50,000 megacycles, of the bandflin which the filter is designed to operate, namely 45,000 to 55,000 megacycles, inclusive.

As indicated in Fig. 6A the left ends of the guides 60! to 620, inclusive, constitute theinput apertures of the manifold M and are arranged on an are having a radius of 3% inches, the center of curvature of which is the mid-point of the output end (right) of input device 600. Similarly the right ends of the guides SDI to 620, inclusive, constitute the output apertures of the delay manifold M and are also arranged on an are having a radius of 3% inches, the center of curvature being the center point of the are upon which the output devices 950 to 559, inclusive, are arranged. The width of the manifold M and the output region 640 are both 3% inches as shown in Fig. 6A.

The outputdevices or horns 550 to 659, inclusive, are similar to the input device 600, but only the lower side of device 650 and the upper side of device 659, as shown in Fig. 6A, are extended to the sides 642 and 644, respectively, of the delay manifold M. The other sides of devices 650 to 659, inclusive, are merely extended until they join those of the next adjacent device on each side as shown in Fig. GA. Top and bottom parallel plates 646 and 648, as shown in Fig. 63, complete the enclosure of output region 540, the distance between the plates 646 and 648 being f g inch. The output devices 650 to 659, inclusive, are also arranged on an are having a radius of 3% inches as shown in Fig. 6A, the center of curvature of which are is the center point'of the above-described are upon which the output apertures of delay manifold M are located.

The over-all filter of Figs. 6A and 6B should be constructed of highly conductive material such as silver or copper. Sheet material inch thick is suitable. Alternatively, to conserve in the use of an expensive material such as silver, 2. plastic, wooden or base metal structure of the required dimensions, having its inner surfaces plated or veneered'with the more expensive material can be used. In manystructures, for example, plywood sheet material the appropriate surfaces of which are painted with metallic paint or covered with thin sheet copper, or the like, are employed.

The filter of Figs. 6A and 6B serves to divide the frequency region between 45,000 and 55,000 megacycles into ten bands, each, approximately, 1,000 megacycles in width, the tenbands being centered about the frequencies, 45,500, 46,500, 47,500, 48,500, 49,500, 50,500, 51,500, 52,500, 53,500 and 54,500 megacycles for the output devices'650 to 559, respectivelv, as indicated in Fig. 6A.

In Fig. 7 a modification of the form of the filter of Figs. 6A and 6B is shown, in which the input and output regions are consolidated into a single duplex region employed for b'oth purposes and the wave-guide delay manifoldcompriseszq sections of wave guide 10: to- 720, inclusive, which are short-circuited at their respective ends which are remote from theduplex;region,as shown.

The input aperture we is, in this structure, the uppermost of the row of apertures of which oisptroxaaeciiinu' e ae ssm i a bands i} being: cc tered about the frequencies 16.0. .0; 75999;; ;Q01L 90Q 75 511000} 2.0 0. ao oian rskto pi es eive r a ,Since th waves"travel ,fromthe apertures of the uid$ 10 .1 t ;l i 11 to: t short-circuited rightendsof'each guide and are their reflected back to the apertures, 7 the waveguide lengths need be only substantially one half ofjth ejlengths required-for the device o f'Figs. SAjand' 6B, Likewise the increase in length of each waveguide in the inanifoldwith respectto the; next sho rter 'wa veguide needs to be only onegalf that employed with the device of Figs. 6A; and 6B,' namely0.65 inch (instead of 1. inches) 1 Otherwise the dimensions and'general construction of the components; can be substantiaily 'th'ose' shown for eorriesponding features of the device 'ofjFigspfi'nfand 6B and described abovel The lengths 10f wave guide comprising the manifold of thefdevice of 'Fig. '7 are computed on the "basis of producing the'maximum response in the 50,000, megacycleoritput when the fre- The devices of Figs. 6A, fiB and 7, obviously, have physical dimensions throughout, which render them thoroughlypracticable from the standpoint of actuallyconstructing them, whereas structuresof the prior artgfor example, those comprising resonant cavities or chambers having critical dimensions which; are in the order of a half wavelength, become impossible to construct, asa practicalgmatter, when the frequencies to beemploy'ed arevery high. For example, at a frequency f 50,000 megacycles, a wavelength is 6 millimetersffand a half wavelength is 3'millimeter's' soth'at prior art devices employing resonant cavities or chambers cannot asa practical matter, be readily maintained at the precise physical dimensions required.

Before proceeding to: describe various combinaquen y of the input energy is also 50,000 megations oftheabove embodimentsfurther, we will the remaining apertures 130 to I38, inclusive, H

are output apertures for ninefrequency bands discuss means whereby amplitude-versus fr'equency characteristic with flatter tops and steeper sides can be provided by microwave filters of :theiinvention,

'(c) More general characteristics through control I .of'phase ues ay manifolds discussedso far have been described *as assemblies of simple transmission lines. 7 These lines can, alternatively, be paths in free space, as in optical spectroscopes, or they an bepathsbetweenjparallel conducting plates,

or in speciallyfldesigned'coaxial lines or wave guides-I I 7 "In anyof these cases, the-delay is constant or nearly constant'as frequency varies, for which it follows that the amplitudewersus-frequency characteristics'as given; for example, by Formula-1L2 and-plotted mug 3 and 4 will have rounded tops and relativelyf gradually sloping sides.

- Ifa high percentage 'oftlie available frequency space is to beflu'seclv in a microwave system empioying several component bands, it may be necessary to employ filters whose amplitude-versusfrequ'ency' characteristics have flatter tops and steepersides. In'the prior microwave filter art, as in the classical low frequency' lumped-element filter artjsuchffilter' characteristics are obtained by emplbyingrnore than one resonance for each channel filter to produce discrimination. An analogous procedure can be followed in designinggmicrowave filt rs of the present invention. The significantly. new factor, with respect to microwave filters of the invention, which is thus added is thatof non-linear phase variation and. the significantly new result which is obtained is that of flatter-topped, steeper-sided amplitude-versus-frequency characteristics.

The dotted line-curve 000 in Fig. 8 represents the linear phase variation of a typical section of a line in a delay manifold which would produce the amplitude-versus-frequency characteristics plotted in Fig. 4. If by some means we can produce,.at microwave frequencies, a phase-versusf-requency characteristic of the stepped type illustrated by the solid curve 802 of Fig. 8, and if structures madeot sections of this type are employed to replace the lines in the manifold which produced the curves plotted in Fig. 4, the resulting -amplitude-versus-frequency characteristics willbeotthe type. illustrated by curves 000 to 904, inclusive, as plotted in Fig. 9-. We see that these curves have fiattertopsand, stepper sides than the curves of Fig. 4. q

The typicalmicrowave line section to produce a phase characteristic of the type plotted as the solid line 002 of Fig. 8 can be'regarded as the microwave equivalent of a low-frequency, lumped-element, all-pass network. Any type of all-pass network, or microwave equivalent thereof, with the required characteristics can in principle be. used. Microwave equivalents of allpass networks, aredescribed in applicants copending application Serial No. 789,985, filed December 5, 1 947, whichmatured into United States Patent 2,531,447, granted November 28, 1950, and in the copending application of D. H. Ring, Serial No. 68,361, filed December 30, 1948, "which matured into United 'States Patent 2,633,492, granted March 31, 1953.

Inthe assembly of Fig. 10, one specific form for a microwave filter of the invention, employing a pluralityof all-pass networks of the type disclosed and described in detail in the abovementioned application of D. H. Ring, is shown.

In Fig. 10 blocks I000 to IOI0, inclusive, are microwave equivalents of all-pass networks according to D. H. Rings abovementioned application, having phase characteristics of the type illustrated by curve 802 of Fig. 8 of the present application. Device I000 is an input device which can be similar to the input device I of Fig. l. The left ends of wave guides I020 to I030 inclusive constitute an array or row of input apertures similar to apertures A1 of Fig. 1 but arranged on an are having a radius I062, the center of curvature I06I of which is the mid-point of the output aperture of input device I060. Radius I062 is preferably made approximately equal to the vertical height I063 of the row of input apertures. The right ends of wave guides 2020 to I030, inclusive, connect to the input terminals of microwave all-pass networks I000 to i I0, inclusive, respectively.

The left ends of wave guides I040 to I050, inclusive, are connected to the output terminals of devices I000 to I 0I0, inclusive, respectively. The right ends of wave guides I040 to I050, inclusive, constitute another array or row of apertures and their arrangement with respect to the five out-put devices I010 to I014, inclusive, is similar to that illustrated in Fig. and described in connection with that figure, above. As for Fig. 5, in Fig. 10 thearc upon which the right ends of wave guides I040 to 1050, inclusive, are located and.- the are upon which the left ends of ill output devices I010 to 1015,. inclusive, are 10- oated have equal radii I005 which'are substantially equal to the 'verticalheight I070 of the array or row of apertures constituting the right endsof the wave guides. 'The center of curvature of each arc is the center point of the other are. As stated above the over-all arrangement of Fig. 10 will divide a broad band frequency input. into five narrow bands, having amplitude frequency characteristics as illustrated, for example, by curves 900 to 904, inclusive, of Fig. 9.

(h) are general characteristics through conjugate combinations oj simple microwave filters The method described in the previous section yofobtaining more general amplitude-versusfrequency characteristics in a microwave filter is applicablewhere appropriate all-pass networks canbe constructed. However, the construction ofan all-pass network, as so far understood in the microwave art, is generally analogous to the construction of a low-frequency lumped-element all-pass network. It requires the design of numerousdevices having specified resonances in specified frequency relationships to each other. The difiiculty of constructing and adjusting such structures increases to the point where they may become impracticable at extremely high frequencies, i. e., at very short wavelengths. I Accordingly we can also well look for a purely distributed circuittechnique for obtaining more general very high. microwave frequency filter characteristics. One such technique will be described in this section.

Fig. 11 shows in block schematic diagrammatic ,form, two microwave filters of the invention,

which can be, for example, two filters of the general typeillustrated in Figs. 6A and 6B, and described in detail above. v

I Microwave filter No. 1, designated H00, and microwave filter No. 2, designated H20, can be identical, and as stated above, can be of the type illustrated. in Figs. 6A and 63.

For filter H00, an input device H02 is adapted to introduce a broad band of microwave frequencies into the filter. The filter H00 will then subdivide the broad band input into a plurality of smaller frequency bands which will be directed into a like plurality of output devices represented by output devices [I04 to H I5, inclusive, respectively. By way of example, the filter can be de- ,signed,-as described in connection with Figs. 6A

and 613, to divide the frequency range extending between 43,500 megacycles and 55,500 megacycles into twelve bands of approximately 1,000 megacycles each, the mid-frequencies of the bands being 4.4,000, 45,000, 46,000, 47,000, 48,000, 49,000, 50,000, 51,000, 52,000, 53,000, 54,000 and 55,000 megacycles, respectively.

Similarly, for filter H20, an input device N22 is adapted to introduce the same broad band of microwave frequencies, as for filter H00, into the filter and filter H20 is adapted to subdivide the broad band input into the same plurality of smaller frequency bands, as for filter II 00,

and to direct these bands into a like plurality of output devices H24 to H35, inclusive, respectively, in the same manner as filter 1 I00 directed them into its output devices.

However, in the arrangement of Fig. 11, the broad band of frequencies is to be introduced only. into the input device H02 of filter H00. A portion of the output devices of filter H00, namely output devices H08 to H12, inclusive,

.are then connected directly to the corresponding output devices H28 to H32, inclusive, respectively, of filter H20 and the input device I I22 of filter H20, now becomes an output terminal for the combination of the two filters H00, H20, shown in Fig. 11, and the five smaller frequency bands, normally appearing at outputs H08 to Hill, inclusive. of filter H00, will, obviously, appear together in the output device H22 of filter H20. The discrimination of the two combined filters against frequencies not falling within any one of the five smaller bands will obviously be the square of that of either filter alone. Assuming, by way of further example, that the twelve bands mentioned above issue from outputs II 04 to III 5, inclusive, with the highest band issuing from H25 and the others in order from the successively adjacent outputs, respectively, the specific combination shown in Fig. 11 would pass the band of frequencies between 46,500 and 51,500 megacycles to filter H20 and output device H22.

The corresponding outputs of the two filters H and H29 which are joined together canbe those of adjacent sub-bands, in which case the band appearing at device H22 will be a' continuous band of frequencies extending from the lowest frequency of the lowest band to the highest frequency of the highest band. Any'number of corresponding outputs can. obviously be 7 interconnected so that any band width from that of a single channel up to that of the broad band introduced into device H02 can be obtained'at output device H22. In the latter case the over-all device would function merely to elimiably designed filters of the invention if increased frequency discrimination is desired.

The bands transmitted from filter I I02 to filter I need not be adjacent. For example, we can connect outputs H95, H05, HM and I H5 of filter H00 to outputs H25, H25, H34 and H35, respectively of filter I I29, leaving all other outputs between these filters H03 and I I20 disconnected,

and there would then. appear in device I I22, two

double bands comprising the joined hands of outputs Her and H (53,500to 55,502 megacycles) and the joined bands of outputs I I I4 and Hi5 (43,590 to 45,500 megacycles), respectively. The discrimination afiorded by the over-all combination for'both of the double bandswould, of course, be the square of that afforded by either filter, alone. An additional microwave filter or filters could then be employed to direct these double bands from output H22 into separate utilization circuits.

From the above, it is apparent that a large number of combinations and permutations of arrangements can readily be devised, employing the principles illustrated by the diagrammatic showing of Fig. 11. Any width of frequency band within the limits indicated above and several frequency bands of difiere'nt or like widths, separated by any of numerous readily determinable frequency intervals can obviously be caused to appear in output device H22, with largely increased discrimination against frequencies out- H08 to EH2, inclusive, then all of the side the passed bands of frequencies, as may be desired to meet predetermined design requirements. Several band widths obtainable in this manner are illustrated by the amplitude-versusfrequency curves I 20I I202, and I203 of Fig. 12, the number of outputs which are interconnected being indicated for each curve. Since the unit microwave filters H00 and H20, are, as discussed above, of a general type which has practicable physical dimensions for frequencies much higher than those for which filters depending upon resonant cavity technique can be constructed, as a practical matter, the importance of such structures of the invention is outstanding.

Let us analyze mathematically the structure which is represented schematically by Fig. 11. This structure consists, as stated above, of two microwave filters of the invention of the general type represented by those described above, in connection with "Figs. 6A and 6B, placed back to back, with some of the output apertures of one portion connected to the corresponding output apertures of the other, the input H22 of the second filter H20 being used as the output of the composite structure.

Let us assume, as illustrated in Fig. 11, that these connections are consecutive, such that each output of the first filter I I00 from H08 to III2 is connected to the corresponding output of the second filter H20 from I I23 to I I32, inclusive, respectively. Let us assume that both filters are lossless and identical. Reasonably close approximations to these assumptions can be attained with this type of filter in actual practice. Let us assume also that at all relevant frequencies within the combined frequency bands of the outputs list to III 2, inclusive, all of the power entering the input I m2 of the filter H00 appears in the interconnected outputs H08 to III 2 inelusive (so that even in transition regions between channels the sum of the output powers is equal to the input power). Now let us examine the transmission between input H02 and output I I22. At frequencies such thatnone of the power entering input I I02 appears in any of the outputs I I23 to i I I2, inclusive, obviously no powerwill be transmitted from input H02 to output I I22.- Atv frequencies such that all of the'power entering input I $22 appears in one or more of the outputs, power will,

. obviously, appear in output i l22. This can be proved as follows Where all of the power P entering input H02 appears. in one or more of the outputs I I08 to I I I2, inclusive, designated by the numerals l to '5,

inclusive, respectively, in thefollowing equations, we have q 3=Q V P1, n+0 Assuming, as will be true for identical filters, that output and input impedances of the two filters are alike (an assumption whichsimplifies the mathematics but is not really necessary to prove the point) and that the input voltage is V 1, then the output voltages are The second filter I I20 is, as stated above, assumed to be'identical to thesfirst filter. Fromthis, and 

